One of two mechanisms may account for the disappearance of naive PC12 pheochromocytoma cells implanted into brain and the retention of PC12 neuronally differentiated by ras-oncogene. As there appears to be no significant difference in major histocompatibility complexes between naive and ras-PC12 cells, it is likely that cell surface changes in, e.g., sialic acid, and cell adhesion molecules (CAMs) may modulate cell-cell and cell-substrate adhesion in vivo and thence the persistence of the ras cells in brain. The amount of sialic acid is assessed by the binding of the lectin, wheat germ agglutinin (WGA), on the surface of naive, nerve growth factor (NGF) treated and ras-PC12 cells. The ras cells have significantly more (125)I-WGA binding than have naive or NGF treated cells. Moreover, WGA blocks regeneration of neurites in NGF exposed PC12 cells but not in ras-PC12 cells. Partially characterized clones of PC12 cells that do not bind WGA, fail to respond morphologic-ally to both NGF and ras treatment. A significant rise in immunoreactivity of protein kinase C, that may phosphorylate proteins associated with the surface molecules, has been detected in PC12 cells 24 hr after infection with ras. Further analysis of surface molecules may support the inference that CAMs rather than histocompatibility complexes affect PC12 cell survival in brain.